Dynamics of the fungus Beauveria bassiana
Beauveria bassiana (Balsamo) Vuillemin is an entomopathogenic fungus virulent towards a variety of insects and is capable of forming an endophytic relationship with corn plants (Zea mays). Studies were performed to determine, the role of rainfall and level of crop residue in fungal transfer, level of infection by naturally occurring B. bassiana in Diabrotica spp. at emergence, and the efficacy of whorl and pollen-shed stage applications of a granular formulation of B. bassiana for control of adult western corn rootworm, Diabrotica virgifera virgifera. Crop residue levels in a raindrop tower study were 0.53, 34, 59, and 84 percent. Simulated rainfall at an intensity of 73 min per hour caused a significant increase in the mean number of B. bassiana colony forming units (CFU) on surface of corn. Plants receiving rain had a mean (+/-SEM) of 8.8 +/- 2.8 CFU per plant; those that did not had an mean of 0.03 +/- 0.01 CFU per plant. The milligrams of soil collected from the surface of plants also was significantly influenced by rainfall. Plants receiving rain had a mean of 15.7 +/- 1.7 mg of soil per plant while those that did not had an mean of 3.4 +/- 0.4 mg of soil per plant. Linear regression showed a highly significant negative relationship between the mean milligrams of soil and CFU per plant on the four levels of crop residue. Field results were similar to those found in the raindrop tower. The number of CFU and milligrams of soil per plant were both higher in conservation-till plots than in no-till plots. The effects of a B. bassiana application at different plant growth stages (whorl stage, pollen shed) and plant surface exposed (leaves and leaf collars; silks; leaves, leaf collars, silks) on levels of infection in D. virgifera virgifera were investigated. In addition, the amount of B. bassiana present in leaf collars was quantified. In the 3 yrs (1998--2000) of the study, application of B. bassiala during whorl stage did not significantly increase the level of infection in corn rootworm beetles. Beauveria bassiana applied at pollen shed in 1998 resulted in 43% beetle infection. Applications at pollen shed in 1999 and 2000 did not result in increased levels of infection. The plant surface to which beetles were exposed did not influence the level of beetle infection. Quantification of B. bassiana in leaf collars showed that fungal application at whorl or pollen-shed stage significantly increased the number of CFU per leaf collar. Diabrotica spp. had low (0%--3.2%) infection with B. bassiana from the soil. Beetles from five of the seven fields surveyed were infected, indicating that natural inocula of B. bassiana active against Diabrotica spp. are widely spread.